1. Field of the Invention
This invention relates to a process for the simultaneous production and purification of 4-nitro-o-phthalic anhydride from an impure mixture containing the same which comprises subjecting said impure mixture to azeotropic dehydration with selected azeotroping agents.
2. Description of the Prior Art
Converting pure 4-nitro-o-phthalic acid to 4-nitro-o-phthalic anhydride is not difficult. This can be done thermally for example, by merely heating the pure 4-nitro-o-phthalic acid at ambient pressure at a temperature of about 164.degree. to about 165.degree. C. for 3 to 4 hours. Water will be driven off and the desired pure anhydride will be obtained. Such conversion can also be effected chemically, for example by heating the pure 4-nitro-o-phthalic acid together with acetic anhydride under reflux. Water and acetic acid will be removed overhead, leaving behind desired anhydride. Acetic anhydride, however, is expensive, since it is consumed in the process. Moreover the use of such technique is not without some difficulty: after contact with excess acetic anhydride (reflux several hours), acetic anhydride is removed by atmospheric distillation and excess acetic anhydride is removed by relatively high vacuum (15 millimeters of mercury) distillation. Pot temperature may not exceed 120.degree. C. or discoloration/charring of the product may result. The resulting product may then require post purification.
However, while such procedures can also be employed to convert 4-nitro-o-phthalic acid to the corresponding anhydride from an impure mixture containing said acid, the desired anhydride will still be present in admixture with the original impurities. In such cases, costly and difficult procedures would be necessary to obtain pure 4-nitro-o-phthalic acid, for the impurities would have to be removed from the impure mixture containing 4-nitro-o-phthalic acid prior to dehydration or from the anhydride mixture containing the impurity after dehydration.